This invention is directed to improvements in Stirling engines and can be used in other Stirling machines, such as Stirling coolers and may have utility as a heat exchanger in other specialty machines. More particularly the invention is directed to improvements in heat exchangers used in these machines.
FIG. 1 is a diagram of a free piston Stirling engine driving an electrical alternator to supply electrical power. As well known to those skilled in the art of free piston Stirling engines, the engine has a displacer 10 that reciprocates in a cylinder 12 and a reciprocating power piston 14 that drives the magnets of the alternator 16. An expansion space 18 opens into an end of the cylinder 12 in the head 20 at the hot end of the engine. The engine has a heat accepting heat exchanger 22 that is adjacent to, and opens into, the expansion space 18 and a heat rejecting heat exchanger 24 adjacent to, and opening into, a compression space 26. These heat exchangers 22 and 24 extend around the cylinder 12 immediately inside of, and in thermally conductive contact with, an outer casing 28. The heat exchangers 22 and 24 are connected to, and open into, opposite ends of a regenerator 30.
Working gas flows in alternating directions between the expansion space 18 and the compression space 26 through the series-connected compression space heat exchanger 24, the regenerator 30 and the expansion space heat exchanger 22. The purpose of Stirling machine heat exchangers is to transfer heat to or from the working gas. For a typical Stirling engine, a heat source, such as a gas flame, is applied to the head 20 for supplying the heat energy that drives the engine. The purpose of the expansion space heat exchanger 22 is to transfer heat from that heat source into the working gas within the engine as the working gas flows in alternating directions through the expansion space heat exchanger 22.
FIG. 1 shows the location of the heat exchanger 22 according to the invention positioned in an otherwise conventional Stirling engine. That heat exchanger 22 has a regenerator end that is connected to the regenerator 30 and an expansion space end that is connected to open into the expansion space 18.
In order to have a high heat transfer rate from the heat exchanger to the working gas flowing through the heat exchanger, it is desirable to have a large number of gas passages that are small in cross section in a plane that is perpendicular to the gas flow direction through the passages. That configuration provides a larger total surface area in contact with the gas for facilitating heat transfer. However, heat exchangers of the prior art that have sufficiently small passages are very costly if those passages are machined by conventional machining tools and techniques because of the difficulty of machining so many passages that are as small as needed. Folded fin heat exchangers have also been used but the passages cannot be made sufficiently small. Sometimes folded fin heat exchangers have been partially crushed to reduce the passage size. But this crushing makes the passages non-uniform in their cross sectional area and therefore non-uniform in flow resistance and heat transfer rate.
One purpose of the present invention is to provide an improved heat exchanger that has thin, narrow gas passages through the heat conducting metal of the heat exchanger without having to machine thin narrow passages. Embodiments of the invention have sufficiently small gas passages but do not require the machining of passages that are as small as the gas passages in the completed heat exchanger.
Another purpose of the invention is to provide a heat exchanger that has a simple structure, is relatively easy to machine and to assemble and therefore has a lower manufacturing cost.
Yet another purpose of the invention is to provide a heat exchanger with very few parts and very few part connections and joints which results in a lower cost heat exchanger that has improved reliability and durability.
Unlike some prior art heat exchangers that require a separately manufactured manifold for interconnecting the heat exchanger to the regenerator, the heat exchanger of the invention can be cast or machined with annular shoulders or other interfacing edges at an end that can fit directly against the regenerator and consequently eliminate the need for a separate manifold.